“These buildings were incredibly strong, especially with respect to resisting dead loads and wind loads, but they also had a number of vulnerabilities,” he said.

Mr Hamburger was a member of the engineer team hired to assess the performance of the World Trade Centre following the disaster.

“The floor trusses were relatively flimsy. As the tower collapsed, the trusses just fell apart,” he said.

He noted that the towers were built using a novel tube frame system that was designed to resist powerful winds of up to 130km/h.

Photographs taken at the World Trade Centre Ground Zero. Picture: Nathan EdwardsSource:News Limited

While the frames were resistant to wind, they were otherwise very weak and broke apart instantly upon impact.

During his speech, Mr Hamburger also noted the angle that the planes hit each tower and how that impacted their collapse.

The first plane hit the north tower at a 45 degree angle and damaged floors 92 to 95. Approximately 40 minutes later the second plane hit the south tower, impacting floors 78 to 84.

“I believe that the hijackers flew the aircraft into the lowest part of the buildings they had access to,” Mr Hamburger said.

“If there had been no nearby structures, they would have hit the towers lower.”

He added that he believed the pilots intentionally flew the planes at an angle in an attempt to cause as much destruction as possible.

Mr Hamburger also said the extreme heat from the fires that broke out caused the steel holding the buildings together to warp.

“The impact probably caused a failure of the fireproofing in the affected areas. We think that the fuel ignited several floors in the building,” he said.

“Steel is born of fire. As it’s reheated, it expands and loses its rigidity. Above 1,000 degrees Fahrenheit (538C), it loses a significant amount of its strength.”

The attack resulted in a total examination of building requirements. Picture: Richard Drew/APSource:News Corp Australia

NOT ENOUGH STAIRWELLS

Another design flaw that may have resulted in less people being able to escape was the small number of stairwells.

When the Twin Towers were starting to be constructed in 1968 a new building code had been issued in New York which meant the number of required stairwells was reduced.

As a result, there were only three stairwells in the centre of each tower which ran up the height of each building.

Two of the stairwells in each tower were just over a metre wide, while the third one was one metre and 42cm wide.

After the planes hit the towers the elevators no longer became a viable option for escape, forcing occupants to run to the nearest stairwell.

Getting to a stairwell itself was a harrowing task and for those that did manage to reach one of the three they were then faced with new challenges.

Some of the stairwells were dark, filled with smoke, covered in jet fuel and jam packed full of people trying to escape.

The narrow width of the stairwells also slowed down escape dramatically.

Those fleeing the building also encountered firefighters in full gear trying to go the opposite way to help those still trapped.

The 9/11 attacks resulted in experts examining what could have been done to prevent the issues faced by those trying to escape the buildings.

Since the attack, national building codes were updated to include requirements like providing an additional stairwell or a specially designed elevator built to still function in extreme situations.

Blast-resistant stair-shaft walls are also now required for high rises over 128 metres high and glow in the dark markings are required on all high rise buildings for people to follow if the lights fail.